Problem: It takes 30 minutes for 10 people to paint 10 walls. How many minutes does it take 19 people to paint 19 walls?
Answer: Imagine that each person is assigned one wall, and all 10 people begin painting at the same time. Since everyone will finish painting their assigned wall after 30 minutes, it takes one person 30 minutes to paint one wall. If we have 19 people and 19 walls, we can again assign one wall to each person. Everyone will take 30 minutes to paint their assigned wall. In other words, it takes 30 minutes for 19 people to paint 19 walls.